IOS – Transferring iPhone message history to new Mac

How about restoring from your backup just the Messages Archive folder contents? (Open that folder in the Finder; then choose "Enter Time Machine" from the Time Machine menu in the Menu Bar; then select the most recent backup that has your full Messages history.) On restore, if anything presently in that folder has the same name, you'll be offered the choice to keep old/new/both.

Relatedly, check out this answer: https://apple.stackexchange.com/a/117558/10581

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As far as merging goes, there are two ways: in the Finder, or in the Terminal. Both of these would require restoring your Time Machine backup of ~/Library/Messages/Archive to another, temporary location.

The Finder method is done by holding the Option key while dragging things to a location containing other things of the same name(s). If the destination contains items with the same name(s) as what is being dragged, it'll offer an option to Merge. But if what is being dragged contains items that are older than those in the destination, no Merge option will be offered.

The Terminal method is to use a command-line tool called ditto. This is something like using the command cp (to copy items), but it will automatically merge if there are name conflicts.

According to Apple's engineering department, backups only store the past 200 messages in the search database. The rest are stored for regular viewing, just not in the database.

"What the customer is experiencing is actually considered expected behavior. With the release of iOS 10, customers are now only able to search the last 200 messages with Spotlight or the in-app search. This would include the last 200 sent and received messages.

The 200 searchable messages limit is only after restoring from a backup prior to iOS 10. This is why devices that had the same Messages database and were not restored from a backup when they upgraded to iOS 10 can still search prior to the 200 limit.

If the customer would like to see this function in a different manner going forward, please encourage the customer to submit feedback at apple.com/feedback. “